Two- and three-phase systems are often employed in chemical reactions. In systems involving the use of reactant gas, the gas is normally contacted with the liquid or solid by bubbling it through the two or three phase system. In these systems, particularly with gas-liquid and gas-liquid-solid particulate phases, the bubble velocity and bubble size become very important parameters in determining behavior of the system and its performance. Such characteristics are particularly important with three phase fluidized beds where the gas and liquid phases act as a media for fluidizing the solid phase. The physical characteristics of the bubbles in fluidized beds affect segregation between reacting phases, bed mixing, particle entrainment from a dense bed, gas hold-up and solid hold-up, mass transfer between the gas and the liquid phase and the solid particulate phase, heat transfer to immersed tube exchangers and bed expansion. Particulate catalysts are used in petrochemical hydrocracking processes and with newly developed very active catalysts, conversion is limited by the transfer of hydrogen from the bubbles in the three phase system to the liquid phase.
Presently, unsatisfactory devices are available for measuring bubble characteristics in three phase fluidized bed systems. In situ sensors have been used; however, they require special design to minimize flow disturbances. Systems, which require bubble sensors, tend to operate at high temperatures and pressures; for example, conditions of heavy oil hydrocracking processes Fischer-Tropsch reactors and H-coal units. Presently, there are no commercially available bubble sensors or probes which can be used in the extreme environments of heavy oil hydrocracking or coal liquefaction reactor.
In the process industry, bar level detectors are in use to control the liquid level in liquid containers. The bar level detector is made of a glass bar having an end with a prismatic shape close to 45.degree. angle which, when it contacts water, beam refraction takes place. When the end is in water, no or low light levels are recorded due to refraction and consequent loss of light energy into the liquid. A photodetector may be used to detect the intensity of light reflected back out of the bar probe. When the bar probe is in the air of the tank, total reflection of the beam takes place and in that instance, a significant increase in light intensity impinging on the photodetector is observed. The changes of light intensity transformed into voltage variations in the photodetector provide the information for an accurate control of liquid tank levels.
Many other types of optical probes are known for use in measuring changes in composition of two and three phase systems. Optical fibre probes have been used in a variety of configurations to detect changes in various parameters of compositions of two and three phase systems. An example is disclosed in U.S. Pat. No. 4,240,747. A fibre optic probe of a diameter of approximately 1.75 millimeters is formed into various complex curved shapes. The formed probe is placed in a liquid. Light emerging from the probe is sensed to provide information which represents the refractive index of a liquid. The probe may be also used to detect the presence of gas bubbles in a liquid where a high frequency fluctuation in the signal at the output section of the probe indicates the presence of the gas bubbles. The high frequency variation of the output signal is due to the probe being considerably larger than the individual bubbles in the system, so that many bubbles pass at one time over the probe causing a rapid variation in the output signal of light reflected in the probe.
One of the difficulties with three-phase fluidized bed systems is to provide a system which can properly assess local solid hold-ups in the fluidized bed. Electroconductivity probes have been used which respond to the difference between liquid, solids and gas dielectric constants and conductivities. However, this approach is limited to systems where the appropriate combination of electrical properties allows the evaluation of the various hold-ups. The system is not readily applicable to catalytic hydrocracking of hydrocarbons or coal liquefaction, because the liquid phase has a very low electroconductivity.
Gamma absorption techniques have been used to assess local hold-ups of solids in fluidized bed systems. The probes used in the gamma absorption technique cannot be arranged in a multi-probe configuration with probes spaced one to two centimeters apart without interference, resulting in the gas liquid and solid hold-ups not being measured simultaneously. This system is disclosed in Vasalos, I. A., D. N. Rundell, K. E. Megiris and G. J. Tjatjopoulos, "Hold-Up Correlations in Slurry Solid Fluidized Beds", AIChE Journal, 28, 2, 346 (1982).
The problem with existing techniques in determining local hold-ups in fluidized beds is that only average hold-up values are provided. In an only very limited situations can local hold-ups be determined by electroconductivity probes.
A multi-probe system disclosed in the Proceedings-Volume 2 of the 33rd Canadian Chemical Engineering Conference of Oct. 2, 1983 uses optical fibre probes for measuring characteristics of bubbles in a three phase fluidized bed system. The probes are each of a U-shape; however, their geometry is not provided. The multi-probe system is capable of measuring bubble velocity and bubble cord length by use of four probes at a vertical separation of 1.25 centimeters. A helium neon laser is used as a source of incident light. Perturbations of light transmitted through the individual probes is detected by a photodetector each time a bubble contacts the respective probe. An analog signal from the photodetector is converted into digital information to provide a basis for analysis of the bubble physical characteristics. This use of the multi-probe system to measure bubble physical characteristics does not contemplate the manner in which the multi-probe system may be used in evaluating, simultanesouly, local gas, liquid and solid hold-ups in a fluidized bed system.